netdev - Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF

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Date:   Wed, 5 Oct 2022 15:43:42 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Martin KaFai Lau <martin.lau@...ux.dev>,
        Stanislav Fomichev <sdf@...gle.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
        xdp-hints@...-project.net, larysa.zaremba@...el.com,
        memxor@...il.com, Lorenzo Bianconi <lorenzo@...nel.org>,
        mtahhan@...hat.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        dave@...cker.co.uk, Magnus Karlsson <magnus.karlsson@...el.com>,
        bjorn@...nel.org
Subject: Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW
 offload hints via BTF

On 05/10/2022 02.25, Martin KaFai Lau wrote:
> For rx hwtimestamp, the value could be just 0 if a specific hw/driver 
> cannot provide it for all packets while some other hw can.

Keep in mind that we want to avoid having to write a (64-bit) zero into
the metadata for rx_hwtimestamp, for every packet that doesn't carry a
timestamp.  It essentially reverts back to clearing memory like with
SKBs, due to performance overhead we don't want to go that path again!

There are multiple ways to avoid having to zero init the memory.

In this patchset I have choosen have the traditional approach of flags
(u32) approach located in xdp_hints_common, e.g. setting a flag if the
field is valid (p.s. John Fastabend convinced me of this approach ;-)).
But COMBINED with: some BTF ID layouts doesn't contain some fields e.g.
the rx_timestamp, thus the code have no reason to query those flag fields.

I am intrigued to find a way to leverage bpf_core_field_exists() some
more (as proposed by Stanislav).  (As this can allow for dead-code
elimination).

--Jesper